The present invention relates to an ultraviolet-shielding agent, a method for the preparation thereof and a cosmetic composition compounded therewith. More particularly, the invention relates to an ultraviolet-shielding agent based on a powdery inorganic material, a method for the preparation thereof and an ultraviolet-shielding cosmetic or toiletry composition compounded with the powdery ultraviolet-shielding agent.
As is well known, ultraviolet light as a constituent of sunlight has a strong influence on the living body to cause various undesirable changes, for example, in the human skin sometimes resulting in cutaneous cancers, and also has an influence to cause degradation of organic materials such as plastics. Accordingly, it is conventional that cosmetic or toiletry compositions to be applied to the human skin are compounded with an ultraviolet-absorbing or ultraviolet-shielding agent so as to protect the human skin from the direct influence of the ultraviolet light. Plastic resins are also compounded with an ultraviolet-absorbing or ultraviolet-shielding agent when shaped articles thereof are to be used in sunlight in order to mitigate ultraviolet-induced degradation thereof.
For example, cosmetic compositions, when desired to have a protecting effect against ultraviolet light, are sometimes compounded with an organic ultraviolet-absorbing agent capable of strongly absorbing ultraviolet light, such as oxybenzone and derivatives thereof, derivatives of salicylic acid, benzophenone compounds, derivatives of p-aminobenzoic acid, derivatives of cinnamic acid and the like. These organic ultraviolet-absorbing agents in general have good compatibility with organic ingredients in cosmetic compositions and plastic resins so that they are widely used when the material compounded therewith is desired to retain transparency or translucency.
Besides organic ultraviolet-absorbing agents such as those mentioned above, similar effects against ultraviolet light can be obtained by compounding a cosmetic composition or plastic resin with an ultraviolet-shielding agent which is an inorganic powder having an ability to scatter or diffuse ultraviolet light including zinc oxide, titanium dioxide, talc, clay and the like. In recent years, several organic compounds have been proposed to serve as an ultraviolet-shielding agent including 5-chlorouracil, guanine, cytosine and the like. Further, a proposal is made to use fine flakes of an iron-containing synthetic mica as an ultraviolet-shielding agent.
The above described ultraviolet-absorbing and ultraviolet-shielding agents each have their respective disadvantages and advantages. For example, the organic ultraviolet-absorbing and ultraviolet-shielding agents are generally not quite stable against irradiation with strong ultraviolet light so that no sustained protecting effect against ultraviolet light can be obtained therewith. In addition, some of these organic compounds have toxicity against the human body so that the application field thereof is more or less limited. Inorganic ultraviolet-shielding agents, on the other hand, also have various problems. Titanium dioxide has a high hiding power so that it cannot be compounded in a cosmetic composition or plastic resin desired to have good transparency or translucency although titanium dioxide has no toxicity against the human body. Some of inorganic ultraviolet-shielding agents have a catalytic activity to accelerate degradation of organic materials to decrease the durability of the material compounded therewith and also have an irritating effect on the human skin so that the amount thereof in a cosmetic composition is necessarily limited.
Accordingly, it is eagerly desired to develop a novel ultraviolet-shielding agent based on an inorganic powder free from the above described problems and disadvantages in the conventional inorganic ultraviolet-shielding agent as well as to develop a cosmetic composition capable of exhibiting an ultraviolet-shielding effect to the human skin without the problems and disadvantages in the prior art compositions.